Homogeneous Charge Compression Ignition, or HCCI, combines certain aspects of spark ignition (SI) and diesel compression ignition combustion engines. Unlike a traditional SI or diesel engine, HCCI combustion takes place spontaneously and substantially homogeneously throughout the combustion chamber. As is the case with a compression ignition engine, HCCI may be a lean combustion process. These conditions translate to a lower local flame temperature which lowers the amount of Nitric Oxide (NO) and Nitrogen Dioxide (NO2) (NO+NO2=NOx) produced in the process.
In comparison to a spark ignition engine, the temperature of the intake charge during HCCI operation is often elevated at the beginning of the compression stroke to reach auto-ignition conditions at the end of the compression stroke. The inventor herein has recognized that the ambient humidity of air drawn into the engine during the intake stroke affects combustion temperature via dilution of the charge with material that cannot be oxidized and because the specific heat of water is higher than air. Thus, as humidity increases, to keep autoignition timing constant, requires a higher initial charge temperature. However, the inhibition of hydrocarbon autoignition by water addition is much greater than can be explained by the decrease in system temperature. Some researchers in the field theorize that water addition exerts chemical inhibition on hydrocarbon autoignition kinetics. Use of humidity sensing or estimation may thus enable improved adjustments to a plurality of engine operating parameters to aid in achieving or maintaining HCCI combustion, even when a vehicle may experience varying levels of ambient humidity.